The present invention relates to an image display device, in particular, relates to a high quality image display device suitable to an organic electronics luminescence (EL) display device.
An image display device using an organic EL element has lately been developed. When an organic EL display device using a plurality of organic EL elements is activated by using an active matrix circuit, each pixcel, or image cell, of an EL is coupled with a field effect transistor (FET) implemented by a thin film transistor (TFT) for controlling current supplied to said pixcel.
FIG. 9 shows a circuit diagram of a prior active matrix type Organic EL display system. The system has a plurality of X-direction signal lines 301-1, 301-2, et al, a plurality of Y-direction signal lines 302-1, 302-2, et al, a plurality of power supply lines (Vdd) 303-1, 303-2, et al, a plurality of switching FET's (select FET) 304-1, 304-2, et al, a plurality of current control FET's (bias FET) 305-1, 305-2, et al, a plurality of organic EL elements 306-1, 306-2 et al, a plurality of capacitors 307-1, 307-2, et al, an X-direction drive circuit 308, and a Y-direction drive circuit 309.
A pixcel to be bright is selected by one of the X-direction signal lines 301, and one of the Y-direction signal lines 302, and a switch FET's (select FET) 304 associated with the selected pixcel is turned ON and the capacitor 307 associated with the selected pixcel is charged. Then, the current control FET (bias FET) 305 associated with the selected pixcel is turned ON, and then, the organic EL element 306 associated with the selected pixcel is supplied current through the power supply line 303 according to the image data to be displayed. Thus, the selected EL element emits light.
For instance, when the X-direction signal line 301-1 receive a signal relating to an image data to be displayed, and the Y-direction signal line 302-1 receives a Y-direction scanning signal, the switch FET (select FET) 304 which is selected by the lines 301-1 and 302-1 is turned ON. Then, the current control FET (bias FET) 305-1 is turned ON according to the image data to be displayed, and the current relating to the image to be displayed flows in the organic EL element 306-1, which then emits light.
The light intensity emitted by an EL element in an active matrix type EL image displace system depends upon the current flowing in a current control FET (bias FET), and said current depends upon charge stored in a capacitor which stores signal. This operation is described in A66-in 201pi Electoluminescent Display Panel T. P. Brody, F. C. Luo, et al, IEEE Trans. Electron Devices, Vol.ED-22, No.9, September 1975, pages 739-749.
As described above, the intensity of each pixcel depends upon the current supply capability of a current control FET 305 (bias FET), and voltage stored in a capacitor 307.
The disadvantage of a prior display system thus described is that the light intensity is not uniform and the image quality is degraded, because the current supply capability of the current supply FET's (bias FET) is not uniform.
When gate voltage V.sub.GS applied to a gate electrode of an FET is gradually increased from the lower voltage than V.sub.0 shown in FIG. 10A, the source-drain current I.sub.DS increases suddenly when the gate voltage exceeds said voltage V.sub.0. The value V.sub.0 is defined to be the gate voltage at which the source-drain current begins to flow. The gate voltage which provides ten times of current is defined as the parameter S (V/decade). In practice, the parameter S is defined by the gradient of the curve of the source-drain current and the gate voltage. The parameter S is the minimum around the threshold V.sub.th which provides a channel. The minimum value of the parameter S is called the value S of an FET as shown in FIG. 10B. The smaller the value S is, the larger the increase of source-drain current I.sub.DS is.
The threshold of an FET is not uniform but dispersed for each elements due to contamination and/or lattice defect. Because of the dispersion, the current depends upon each element even for the uniform gate voltage, and the effect of the dispersion is large when the gate voltage is close to the threshold where the value S is small and the gradient of the current is large.
FIG. 10C shows the experimental result of the dispersion. We produced many FET's each of which has an upper gate electrode 100-1, a lower gate electrode 100-5, an upper gate oxide layer 100-2, an active layer 100-3, and a lower gate oxide layer 100-4. The lower gate electrode 100-5 is fixed to the source voltage. The control voltage 0-10 V is applied to the upper gate electrode 100-1, and the source-drain current is measured. The average A.sub.ve of I.sub.DS for each control voltage V.sub.g, and the standard deviation (.alpha.) is obtained. The solid curve in FIG. 10C shows the ratio (.alpha.)/A.sub.ve for each control voltage V.sub.g.
Similarly, when an upper gate electrode is fixed to source voltage, and control voltage 0-17 V is applied to a lower gate electrode, the dotted curve in FIG. 10C shows the similar relation between the control voltage V.sub.g and the ratio (.alpha.)/V.sub.av for each control voltage.
It should be noted in FIG. 10C that the characteristics of an FET disperses much even if the structure is the same, and the deviation or the dispersion is the maximum for the specific control voltage. The control voltage V.sub.g which provides the maximum deviation is the same as the gate voltage which gives the value S, and is close to the threshold.
As described above, a prior active matrix type organic EL display element has the disadvantages that the dispersion of the characteristics when gate voltage is close to threshold is large, and the dispersion of display intensity is large in particular when image is dark.